1. Field of the Invention
The present invention is in the field of shaft kiln assemblies and methods for operating the same and involves an improved apparatus for calcining limestone and the like wherein the apparatus contains a plurality of double walled hollow support means positioned at different vertical levels, and fuel conduits consisting of a bundle of individual pipes with guide means being provided internally of the hollow support means which position the fuel conduits in the appropriate location within the hollow interior of the shaft kiln.
2. Description of the Prior Art
Shaft kilns for calcining limestone and operating with liquid or gaseous fuels are known. Such kilns consist of a shaft which is provided at its top with devices for introducing the lime to be burned and for extracting the combustion gases. Discharge openings through which the cooled lime is withdrawn and through which air is introduced and preheated by means of contact with the product are situated in the floor of the calcining kiln. Hollow supports which contain lines having openings therein for the uniform distribution of liquid or gaseous fuel throughout the calcining zone are located in the calcining zone of the shaft kiln. The hollow supports are so constructed that complete combustion of the fuel occurs in a plurality of stages, and uniform distribution of the combustion heat in the entire calcining zone is obtained. The hollow supports may be equipped with cooling chambers for conducting a cooling fluid and a temperature control means may be provided which controls the kiln temperature according to changes in temperature of the cooling fluid. Such a device is shown in German AS No. 1,156,694 (U.S. Pat. No. 2,933,297).
The known shaft kilns depend exclusively on the employment of gaseous, liquid, or in some cases powdery fuels. Particularly in the case of powdery fuels, localized, undesired temperaure peaks and overburning may occur which are attributed to a too rapid burnout of the coal dust.
Coal dust as defined herein consists of material having particle sizes from the submicron range up to 500 microns.
It has also been known for a long time to calcine limestone in low shaft kilns with fuel in the form of lumps, employing a mixture of limestone and lump coal as the feed. However, these earlier methods face the problem that due to demixing of the feed material and the well-known phenomenon of channelling gas paths over zones of lower resistance, a non-uniform burning within the kiln cross section resulted in the simultaneous production of overburned and underburned material. This phenomenon becomes a serious problem particularly with larger kiln dimensions.